Bias-corrected Antarctic sea-level projections reveal greater impact of historical warming

The Antarctic Ice Sheet (AIS) is the largest source of uncertainty in sea-level rise (SLR) projections, posing substantial risks to coastal communities worldwide. AIS projections are subject to cascading uncertainties propagating through emission scenarios, atmosphere-ocean general circulation models (AOGCMs), ice sheet dynamics, and sea-level physics. In numerical ice sheet model intercomparison projects, these uncertainties—typically attributed to unexplained intermodel differences—stem from modeling choices that may bias ensembles toward commonly adopted approaches regardless of observational consistency. Here we show that, unlike previous ensemble projections, bias-corrected projections indicate it is very likely (P>=0.92) that historical warming has committed the AIS to continued mass loss through the 21st century even under aggressive emission reduction scenarios. Our results arise from a machine learning emulator that systematically quantifies how each physical assumption cascades into spatiotemporally variable SLR uncertainties and enables bias correction through transient calibration. Higher emissions drive greater mass loss within this century (P>=0.89)—affecting both near-term risks and long-term millennial commitments. Notably, under very high emission scenarios, satellite observations cannot rule out up to 25.4 cm of SLR (95th percentile; median = 15.7 cm) from cascading physical mechanisms by 2100. Reducing these substantial tail risks requires rapid emission reductions and improved constraints on AOGCM selection, basal sliding laws, and ice shelf basal melt parameterizations, the primary uncertainty drivers identified by our analysis.